1. Field of the Invention
This invention relates generally to communication devices and, more particularly, to processing of received signals in mobile communication devices.
2. Description of the Related Art
Mobile communication systems typically include mobile communication devices, such as cellular telephones, that transmit and receive telecommunication signals to and from fixed base stations. The fixed base stations may have interfaces to the public switched telephone network (PSTN), also referred to as land lines. The transmission and reception of signals typically occurs in the radio frequency (RF) bands of the telecommunications spectrum at assigned frequencies. The signal power of such transmissions is relatively low, often measured in milliwatts (mW) or decibel milliwatts (dBm).
A base station typically transmits a pilot signal over a control channel. The mobile communication devices monitor the control channel when not actively engaged in a telephone call and provide identification information over the control channel. This permits the base stations to know the identity of communication devices for which they have responsibility. When a call arrives at the base station from the PSTN, the base station sends a control signal for the appropriate mobile device, which then receives the control signal and moves to an assigned call channel to receive the telephone call. Thus, the mobile communications device is continuously receiving either the control pilot signal or an active telephone call signal. Typically, all telephone call signals received at a mobile communication device are assigned to one frequency band, and all telephone call signals transmitted from a mobile communication device are assigned to a different frequency band, higher than the receiving band.
When a cellular base station detects that the signal from a mobile device is too weak, the base station queries a neighboring base station to determine the signal strength from the same mobile device. If the signal being received by the neighboring base station is stronger, then the first base station hands off the telephone call to the second base station, which takes over communication responsibilities for the telephone call. Thus, base stations continuously check the power of signals received from the mobile devices, using subsystems referred to as received signal strength indication (RSSI) systems.
The mobile communication devices also require RSSI systems because the mobile communication devices must also report to the base stations on the strength of the signals they receive from the base stations. The base stations will request RSSI data from the mobile devices and will use that information for making hand-off decisions, power modulation commands, and the like. Yet another application of RSSI systems is to determine which base station or channel to use. The mobile device will measure the signal strength over several channels and use the strongest received signal for processing.
The mobile device RSSI systems are usually calibrated during a preproduction calibration phase of operation, in which signals of known power are transmitted to the mobile device, and the voltage of the received signal is measured in the device and correlated to the known signal transmission power. In a mobile device with digital operation, the voltages of the received signals are stored in an RSSI table that pairs the received voltages with the known signal strength values. A central processing unit (CPU) of the mobile device can thereafter determine the received signal voltage and, if necessary, interpolate between two received signal power entries of the RSSI table to find a signal power value that corresponds to the received signal voltage.
In addition to measuring received signal strength, the mobile devices are required to modulate their respective transmitted signal power in response to the base stations, and therefore must monitor the strength of their own output signals. For example, if two mobile devices are being handled by the same base station, and one is much closer to the base station than the other, the closer mobile device will have a much greater received signal power at the base station if both mobile devices transmit at the same power. In that situation, the signal from the closer mobile device can overwhelm the signal from the farther mobile device, even if they are assigned to different frequencies, and this will disrupt communication.
To prevent disruption of calls, a base station that is handling multiple mobile devices will request a closer mobile device with a stronger received signal to reduce the power of its transmitted signal. In most telephone systems, the amount of power reduction will occur in predetermined, incremental steps, through different power levels. For example, power levels may occur in steps of 4 dB. Those skilled in the art will recognize "dB" as a relative power indication that equates to power rations, where typically xdB (signal power)=10 log (ratio), so that 3 dB represents twice the power, where 3dB=10 log 2, and 10 dB is ten times the power (10 dB=10 log 10) (it should be noted that for relative signal voltage, the relationship is xdB (voltage)=20 log (ratio)). For telephone mobile systems, most power units are in milliwatts, written as "dBm" units, where 0 dBm is 1 milliwatt. Thus, 10 dBm is a power level 10 dB above 1 milliwatt, because 10 dB is a ratio of ten times the power. Thus, 10 dBm indicates ten times above 1 milliwatt, which is equal to 10 mW, and 20 dBm is 100 mW, 30 dBm is 1000 mW (1 watt), and so forth. Thus, it is necessary for the mobile communications devices to control and monitor their respective signal power outputs, and therefore they must have systems that detect their respective transmitted signal power, in addition to their RSSI systems.
Conventionally, power detection of transmitted signals involves systems that include a power measuring circuit having a detector diode and voltage capacitor that takes a sample of the transmitted signal, thereby extracting a portion of the signal power, and then scales up the detected voltage to derive a signal power value. For example, the conventional mobile device power measuring circuitry may tap the output signal and inherently reduce the signal strength Furthermore, the dynamic range of the detector circuitry is limited. Such diode detection systems can be problematic, because they often have multiple diodes, each with different temperature sensitivities and non-linear modes of operation. In addition, the detected voltage (and power) varies over frequency as well. Inaccuracy in transmitted power measurement is the usual result.
Each additional circuit that is required for a mobile communication device adds to the production cost of the device. Additional circuitry also adds to the weight and physical size of the device, which is undesirable in a mobile communication device. Finally, each additional circuit is potentially an added power drain on the on-board (battery) power available, decreasing battery life and duration of charge.
From the discussion above, it should be apparent that there is a need for a mobile communication device that can monitor transmitted signal power and received signal power with a minimum of cost, size, and energy use. The present invention fulfills this need.